Coronary artery plaque morphology was studied in 354 five-mm segments of the 4 major (left main, left anterior descending, left circumflex, and right) epicardial coronary arteries in 10 patients with isolated unstable angina pectoris with pain at rest. The 4 major coronary arteries were sectioned at 5-mm intervals and a drawing of each of the resulting 354 movat-stained histologic sections was analyzed using a computerized morphometry system. The major component of plaque was a combination of dense acellular and cellular fibrous tissue with much smaller portions of plaque being composed of pultaceous debris, calcium, foam cells with and without inflammatory infiltrates, and inflammatory infiltrates without foam cells. There were no differences in plaque composition among any of the 4 major epicardial coronary arteries. Plaque composition varied as a function of the degree of luminal narrowing. Multi-luminal channels were seen in all 10 patients (28 [19%]) of the 146 sections narrowed >75% in cross sectional area and in 36 (10%) of all 354 segments. The high frequency of multi-luminal channels and a high percent of plaques composed of fibrous tissue suggest that a large portion of plaques in patients with angina at rest develops as a consequence of recurrent thrombosis. Key words: coronary artery disease, thrombus, multiluminal channels, plaque rupture.